1. Field of the Invention
The present invention relates to laser-ablation transfer printing processes and laser-induced melt-transfer printing processes. More specifically, the present invention relates to techniques for providing contact between a donor sheet and an acceptor sheet in laser-ablation transfer processes and laser-induced melt-transfer processes, and for conducting laser-scanning in connection therewith.
2. Background Information
Laser-ablation transfer printing and laser-induced melt-transfer printing (collectively referred to herein as laser-induced thermal transfer printing) involve the transfer of a material from a donor sheet to an acceptor sheet to form a representation of an image on the acceptor sheet. During this transfer, it is necessary for the donor sheet and acceptor sheet to be held in contact with one another. The transfer of material is thermally induced by the application of a scanning laser beam at selected points across the donor sheet-acceptor sheet combination.
Laser-induced thermal transfer printing is well known to be useful for producing halftone color proofs, films, printing plates and other printing forms. Specifically, this type of transfer printing is known to be particularly useful for applying an ink-accepting coating onto a seamless sleeve having a hydrophilic surface, and also for applying an ink-repelling material onto an ink-accepting surface. Processes for using laser-induced thermal transfer printing to make printing plates and other printing forms are well known and are described for example in U.S. Pat. Nos. 3,964,389 and 5,819,661, which specifically address laser-ablation transfer printing and laser-induced melt-transfer printing, respectively.
The composition of the donor sheets and acceptor sheets used in connection with laser-induced thermal transfer printing is likewise well known in the art. For example, U.S. Pat. No. 5,757,313 discusses donor elements containing polymerization initiators, and U.S. Pat. No. 5,238,778 discloses donor elements containing photo-curable compositions. U.S. Pat. No. 5,607,810 discloses a peel-apart assembly which can include donor elements having transferable dyes and acceptor elements having non-proteinic hydrophilic surfaces. U.S. Pat. No. 5,401,606 describes a laser-induced melt transfer process in which a melt viscosity modifier is utilized to better facilitate the melt transfer process between the donor and acceptor.
In laser-induced thermal transfer printing processes, it is known that the donor sheet and acceptor sheet must be held in contact with one another with relatively uniform contact pressure across the donor-acceptor combination, to insure uniform transfer characteristics for a specified level of laser energy. In connection with such printing processes, donor sheets and acceptor sheets traditionally have been pre-assembled into a subassembly. The donor-acceptor subassembly has been attached to either an internal drum or an external drum for laser imaging. Once the laser imaging has been completed, the donor sheet and the acceptor sheet have been separated from one another. In printing plate-making applications, the acceptor typically has been used as a printing plate.
For certain laser-induced thermal transfer printing applications, it has been considered desirable to assemble donors and acceptors directly on the imaging device. Where an external drum arrangement has been used in such techniques, the acceptor sheet typically has been first affixed to the outer circumference of the drum, and the donor sheet has then been secured over and substantially coextensively with the acceptor sheet. Certain laser-induced thermal transfer printers of the prior art, such as those disclosed in U.S. Pat. No. 5,446,477, have used vacuum drum arrangements to achieve the requisite sufficiently uniform contact between the donor sheet and acceptor sheet. Such vacuum drum arrangements have added significant cost, size, and complexity to the printers in which they are used, however.
Certain other laser-induced thermal transfer printers of the prior art, such as those disclosed in U.S. Pat. No. 5,764,268, have provided contact between the donor sheet and the acceptor sheet without the need for a vacuum drum arrangement. Such laser-induced thermal transfer printers have utilized dedicated tensioning mechanisms and clamping devices to apply tension to the donor sheet, and to draw the donor sheet into contact with the acceptor sheet.
In addition to laser-induced thermal transfer printing techniques, other types of thermal transfer printing utilizing the assembly of donors and acceptors directly on the imaging device are also well known in the art. For example, U.S. Pat. No. 5,072,671, the contents of which is incorporated herein by reference, discloses an apparatus and method for transferring an imaged donor layer generated by a thermal recording head from an intermediate support to an acceptor via a reproducing means. Specifically, this transfer is accomplished by transferring meltable particles from the donor layer onto a deformable acceptor surface. U.S. Pat. No. 4,958,564 describes a method of using a rigid thermal head to transfer a donor substance from a donor support to an intermediate surface, and of then transferring the donor substance from the intermediate surface to the final acceptor. This patent also discloses the technique of transferring to a rigid printing form the donor substance which remains on the donor support after the above-described transfer of the donor substance from the donor support to the intermediate surface.
U.S. Pat. No. 4,804,975 describes a thermal dye transfer apparatus which absorbs heat from a laser light. Donor and acceptor sheets are hard pressed into close contact in the projection area by a pressure plate.
Therefore, in view of the above-described examples and limitations in the existing art, a need has arisen for further laser-induced thermal transfer printing techniques in which donors and acceptors are assembled directly on the imaging device. A need has also arisen for such techniques which do not require vacuum drum arrangements or dedicated tensioning mechanisms and clamping devices to maintain the requisite contact pressure across the donor sheet-acceptor sheet combination. A need has also arisen for such techniques which eliminate the need for manual separation of donor sheets and acceptor sheets. A need has also arisen for such techniques which eliminate the need for disposal of donor supports once the printing process has been completed, and in which donor supports instead can be recoated with donor material, thereby reducing waste and cost. A need has also arisen for such techniques in which donor sheets can be conveniently supplied on rolls.